The present invention relates to a shutter speed display device for a camera in which light emitting diodes or the like are employed to display or indicate a shutter speed in the viewfinder of the camera.
Devices for activating a common anode or cathode light emitting diode array and for activating light emitting diodes arranged in matrix form, as in Japanese Laid-Open Patent Application No. 129227/1976 have been proposed as display devices for displaying a shutter speed set in a camera in a manual exposure time control mode or for indicating whether a shutter speed has been correctly set or not. The first-mentioned device is disadvantageous in that the number of light emitting diode terminals is considerably large while the second is disadvantageous in that, in order to indicate whether or not the shutter speed is correctly set, it is necessary to provide an additional light emitting diode and to turn it on or to cause it to flicker.
Accordingly, an object of the invention is to eliminate the above-described difficulties accompanying a conventional display device.
FIG. 1 is a sectional view of a portion of a viewfinder adapted to display shutter speeds with light emitting diodes. FIGS. 2 and 3 are circuit diagrams showing the conventional connections of light emitting diodes.
As shown in FIG. 1, numerals 2 indicative of exposure times are arranged in a viewfinder 1 with light emitting diodes L.sub.1 through L.sub.16 disposed adjacent corresponding ones of the numerals 2.
In FIG. 2, all of the anode terminals A of light emitting diodes L.sub.1 through L.sub.16 are connected together. The cathode terminals of the light emitting diodes L.sub.1 through L.sub.16 are designated by reference characters B, C, . . . and Q, respectively. When a negative voltage is applied to the cathode terminal G and a positive voltage is applied to the anode terminal A, the light emitting diode L.sub.6 is activated causing it to emit light so that a shutter speed 1/125 sec is indicated. If the light emitting diodes are operated in such a manner that, in a manual photographing operation, the light emitting diode L.sub.1 is caused to flicker or to be activated to indicate over-exposure and the light emitting diode L.sub.16 is caused to flicker or be activated to indicate under-exposure and the light emitting diodes L.sub.1 and L.sub.16 are turned off and only a single light emitting diode corresponding to a manually selected shutter speed is turned on when a correct exposure setting is obtained, then the light emitting diodes can serve as a manual exposure meter. However, the arrangement of light emitting diodes shown in FIG. 2 is disadvantageous in that, as there are a large number of light emitting diode terminals, it is necessary for a circuit driving the light emitting diodes to have a corresponding large number of pins resulting in a high manufacturing cost and a low dependability of the camera.
In order to eliminate the above-described difficulty, a display circuit as shown in FIG. 3 has been proposed as disclosed in Japanese Laid-Open Patent Application No. 129227/1976 and in U.S. Pat. No. 4,096,494 assigned to the assignee of the present application. In FIG. 3, a decoder and driver circuit 3 is made up of inverter circuits 5 through 8, AND gates 9 through 12 and OR gates 13 through 16, and a display unit 4.
The operation of the display circuit in FIG. 3 will be described. The relationships between four bit inputs R, S, T and U and the outputs V.sub.4, V.sub.3, V.sub.2 and V.sub.1 of the AND gates 9, 10, 11 and 12 and the outputs W.sub.1, W.sub.2, W.sub.3 and W.sub.4 of the OR gates 13, 14, 15 and 16 are indicated in a function table in FIG. 4. For instance, when the inputs R, S, T and U are at "1", "0", "1" and "0", respectively ("1" being a logic high level and "0" being a logic low level), the outputs V.sub.1, V.sub.2, V.sub.3 and v.sub.4 of the AND gates are at "0", "0", "1" and "0", respectively, and the outputs W.sub.1, W.sub.2, W.sub.3 and W.sub.4 or the OR gates are at "1", " 1", "0" and "1", respectively. That is, in this case, the outputs V.sub.3 and W.sub.3 are at "1" and "0", respectively, and therefore only the light emitting diode L.sub.6 is turned on.
This display circuit is disadvantageous in that it can activate only one light emitting diode at a time and cannot perform the desired functions of a manual exposure meter in which a light emitting diode indicating correct exposure or a light emitting diode indicating whether a set exposure time is suitable or not as well as a light emitting diode corresponding to a manually set exposure time are simultaneously activatable.